Power I/O Wildcard User Guide

Maximum Ratings: Current, Power and Switching Frequency

Without using heat sinks the outputs can each sink 2 A continuously. They can also sink pulses of greater current (up to 10 A) as long as the pulse duration and frequency do not cause a greater power dissipation than 2 A of continuous current does.

Owing to the low ON resistance of the MOSFETS the power dissipated in them is low: when OFF they are subjected to the field voltage but there is no current so no power is dissipated; when ON their internal resistance is low (typically 0.15Ω) so the I 2R power is also low.

Without a heat sink, the maximum electrical power allowed for each channel is limited by convection/conduction from the MOSFETís TO-220 package, and depends on the ambient temperature, as shown in Equation 1-1. In the equation, Tjmax is the maximum junction temperature (175°C), TA is the ambient temperature (0-70°C), and RΘ is the junction to ambient thermal resistance (62°C/W). For an ambient temperature increasing from 25°C to 70°C, the maximum power that can be dissipated decreases from 2.4 W to 1.7 W.

Max Power Dissipated = (Tjmax-TA)/RΘ

Eqn. 1-1

The heat generated in the MOSFET comprises two terms, an I²R term and a frequency-dependent switching loss term, as given in Equation 1-2.

Power Generated = I²rds D + F I V (ton + toff)/(6 or 2)

Eqn. 1-2

The first term is the product of the square of the ON current, the device resistance (rds = 0.4 Ω max at the max junction temperature), and the duty factor (0<D<1). The duty factor is the ratio of ON time to (ON + OFF) time if the channel is switched repeatedly. The duty factor is unity, (D=1), for continuous current. The second term represents switching loss incurred during the turn on and off transitions. It is the product of the frequency, F, the ON current and OFF voltage, I and V, and the sum of the ON and OFF transition times (21μsec). There is either a 6 or a 2 in the denominator if switching resistive (use 6) or inductive (use 2) loads respectively. A load is considered inductive if its inductance , L, is greater than a threshold given by L>V toff /4I. For example, if I=2A and V=25V, the load is inductive if its inductance is greater than 25V * 12μsec / (4 * 2A) = 38μH.

The conditions of maximum current, voltage and frequency are found by setting the maximum package dissipation equal to the electrical power as shown in Equation 1-3.

(Tjmax-TA)/RΘ = I 2rds D + F I V (ton + toff)/(6 or 2)

Eqn. 1-3

And substituting the appropriate constants, as,

(175-TA)/62 = 0.4 I² D + 0.021 F I V / (6 or 2)

Eqn. 1-4

in which I is in amps, V in volts, D is a fraction between 0 and 1, F is in kHz, and TA is in °C.

Equation 1-4 can be solved to find any of the parameters (I, V, D, F, TA) given the others.

Example 1, finding maximum DC current: For continuous DC current (F=0 and the channel is ON so D=1) and a maximum ambient temperature of TA=50°C, the maximum continuous current is found by solving Eqn. 1-4 to be 2.2 A. At the maximum specified ambient temperature, TA=70°C, the maximum current is found to be 2.06 A.

Example 2, finding a maximum switching frequency: For driving a 1 A resistive load (I=1) from a field supply of 25 volts (V=25) with a duty cycle of 50% (D=0.5) at a maximum ambient temperature of TA=70°C, we find a maximum frequency, F, of 17 kHz.

Example 3, finding a maximum switching frequency for an inductive load: For driving 1A into an inductive load (I=1) of 1 mH from a field supply of 25 volts (V=25) with a duty cycle of 50% (D=0.5) at a maximum ambient temperature of TA=70°C, we use the denominator of 2 in the above equation and find a maximum frequency, F, of 5.6 kHz.

Example 4, finding duty cycle: PWM modulation of a thermoelectric cooler requires driving 4 A into the cooler at its minimum recommended frequency of 1 kHz from a 12 volt supply. The instrument must operate at ambient temperatures up to 70°C. That is, I=4, F=1kHz, V=6, and TA=70°C. The maximum duty cycle is found as 24%.

Itís wise to stay well within the computed maximums. In addition to these considerations there are transient effects for high current pulses. Pulses of 10 A must be restricted to times less than 50 milliseconds, while 4 A pulses may have a maximum duration of 1 sec. Consult the IRLZ14 data sheet for details.